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FIG. 3. Schematic illustration of C4 and C3 protein structures. 0=C—S represents the reactive thioester bond that permits covalent attachment of both C3b and C4b to surface nucleophiles. The pattern of proteolytic degradation and the resulting fragments are also shown. Although factor I is the relevant in vivo protease, some of these same fragments can be generated with trypsin, plasmin, and thrombin.